Patent GB-1143799-A (GB)
IPC: H03G3/20
Abstract: 1,143,799. Photo-electric flaw detectors. ELECTRONIC ASSOCIATES OF CANADA Ltd. March 14, 1967 [March 21, 1966], No.12025/67. Heading G1A. [Also in Division H3] Two negative feedback loops are provided in the circuit of a flaw detector, one effective to reduce very low frequency variations due to drift, caused e.g. by accumulation of dirt &c. on the sensor, ageing of the sensor or changes in power supply, and the other effective to reduce higher frequency variations due to noise, caused by the scanning system. Flaws in the material under investigation produce fluctuations in sensor output of higher frequency than even the scanning noise, and are thus detectable. Amplitude discrimination circuitry can be provided in one or both feedback loops to prevent transmission of pulses of lower amplitude than the bias. In one arrangement, Fig.1 (not shown), light from a lamp (16) powered by supply unit (38) is transmitted through the material (14) under analysis and detected by a sensor (12), the output of which is amplified by a wide band (0-100 Kc/s) amplifier (24) passing all frequency variations to be encountered and feeding detector circuitry (26) sensitive only to fluctuations due to flaws (20-100 Kc/s) which includes pulse amplitude discriminators, counters and recorders. Two feedback loops are provided to control the power supply unit (34) for the sensor (12). One includes amplifier (28) passing frequencies from 0À06-20 Kc/s due to scanning noise, and the other frequencies from 0-0À06 Kc/s due to drift. The latter loop may be provided with an amplitude discrimination circuit (36). In a modification, Fig.2 (not shown), the amplitude discrimination circuit (36) is common to both feedback loops, and the loops control the power supply (38a) of the source (16) as well as that of the sensor (12). The embodiment of Fig. 4 shows a scanning system for investigating a material 14 by reflected light. The circuit includes means for varying the amplitude discrimination level in the noise-correcting feedback loop. The material 14 is moved past a lamp 16 by motor 18 and is viewed by a scanning system comprising a six-sided mirror drum 15 rotated by motor 18 and directing light through lens 17 and aperture 21 on to the photo-cathode of a photo-multiplier 12. Thus the width of the material is scanned at a fast rate e.g. 360 scans/ sec. The output of the photo-multiplier 12 is gated to the flaw detector unit 26by circuit 46, when the field of view of the photo-multiplier 12, as determined by aperture 21, is within the edges of the material, under the control of a commutator 48 rotating synchronously with the drum 15. The feedback loops are constituted respectively by the circuit including triodes 58, 74,and 66, capacitor 76 and discharge tubes 88 and 68, correcting for drift, and the circuit including transistors 92, 94, pentode 96 and capacitors 106, 108 correcting for scanning noise. The diode 124 clamps the anode of the photo-multiplier to earth. Emitter follower transistors 54, 56 cause the grid of triode 58 to reproduce at a low impedance level the voltage variations at the anode of the photo-multiplier 12. Only when the anode of triode 58 is greater than a predetermined potential is diode 65 allowed to conduct through triode 74 causing the current through high voltage triode 66, connected to the triode 74 through a corona discharge tube 88, to be reduced. Since triode 66 controls the supply to the photo-multiplier 12, increased conduction through the triode 66 reduces the potential across the photo-multiplier, decreasing the output current of the latter. The transmission frequency of this loop is kept to zero and low frequencies by capacitor 76. The output of emitter followers 54, 56 is also fed through voltage divider network 112, 114 to transistors 92, 94 controlling pentode 96. The setting of variable resistor 112 determines the bias in this loop and thus the level of amplitude discrimination in comparison with the fixed level set by diode 65 in the loop. D.C. blocking capacitor 106 and capacitor 108 determine the pass-band of this loop.